Chemical kinetics modelling study of naturally aspirated and boosted SI engine flame propagation and knock

Modern spark ignition engines are downsized and boosted to meet stringent emission standards and growing customer demands on performance and fuel economy. They operate under high intake pressures and close to their limits to engine knock. As the intake pressure is increased knock becomes the major barrier that prevents further improvement on downsized boosted spark ignition engines. It is generally accepted that knock is caused by end gas autoignition ahead of the propagating flame. The propagating flame front has been identified as one of the most influential factors that promote the occurrence of autoignition. Systematic understanding and numerical relation between the propagating flame front and the occurrence of knock are still lacking. Additionally, knock mitigation strategy that minimizes compromise on engine performance needs further researching. Therefore the objectives of the current research consist of two steps: 1). study of turbulent flame propagation in both naturally aspirated SI engine. 2) study of the relationship between flame propagation and the occurrence of engine knock for downsized and boosted SI engine. The aim of the current research is, firstly, to find out how turbulent flames propagate in naturally aspirated and boosted S.I. engines, and their interaction with the occurrence of knock; secondly, to develop a mitigation method that depresses knock intensity at higher intake pressure. Autoignition of hydrocarbon fuels as used in spark ignition engines is a complex chemical process involving large numbers of intermediate species and elementary reactions. Chemical kinetics models have been widely used to study combustion and autoignition of hydrocarbon fuels. Zero-dimensional multi-zone models provide an optimal compromise between computational accuracy and costs for engine simulation. Integration of reduced chemical kinetics model and zero-dimensional three-zone engine model is potentially a effective and efficient method to investigate the physical, chemical, thermodynamic and fluid dynamic processes involved in in-cylinder turbulence flame propagation and knock. The major contributions of the current work are made to new knowledge of quantitative relations between intake pressure, turbulent flame speed, and knock onset timing and intensity. Additionally, contributions have also been made to the development of a knock mitigation strategy that effectively depresses knock intensity under higher intake pressure while minimizes the compromise on cylinder pressure, which can be directive to future engine design

Modal test and finite element analysis of a turbine disk

Experimental modal analysis of a turbine disk was conducted with the hammering method. The first five modals were obtained, matches well with calculation results of ANSYS, and proves the effectiveness of the experiment, provides a reference for further improvement of a certain engine

Modelling of transient stretched laminar flame speed of hydrogen-air mixtures using combustion kinetics

The calculations of laminar burning velocity are mostly based on empirical correlations obtained from combustion bomb experiments. There is a noticeable scarcity of the fitting parameters in these correlations, especially under increased temperature and pressure conditions. The effects of flame stretch and instabilities further complicate the situation as these effects are not distinguished in some correlations. Furthermore, although combustion products are of great interests in recent computer simulations of combustion, it is difficult to integrate combustion chemistry into the existing correlations. This paper discusses a laminar burning velocity model for hydrogen-air mixtures in a constant volume combustion bomb. The model is based on a one-dimensional three-zone thermodynamic model that calculates the mass transfer and diffusion and the heat transfer between zones. The chemical process involved in the combustion is solved by an in-house chemical kinetics solver with an established reduced hydrogen-oxidation mechanism from literature. The effects of flame stretch and instabilities are simulated using existing experimental data. The calculated laminar burning velocities are compared to existing empirical correlations and experimental data obtained from constant volume combustion bomb tests. The model is able to simulate laminar burning velocities and have the potential to be integrated into IC engine models in the future

Modelling of transient stretched laminar flame speed of hydrogen-air mixtures using combustion kinetics

The calculations of laminar burning velocity are mostly based on empirical correlations obtained from combustion bomb experiments. There is a noticeable scarcity of the fitting parameters in these correlations, especially under increased temperature and pressure conditions. The effects of flame stretch and instabilities further complicate the situation as these effects are not distinguished in some correlations. Furthermore, although combustion products are of great interests in recent computer simulations of combustion, it is difficult to integrate combustion chemistry into the existing correlations. This paper discusses a laminar burning velocity model for hydrogen-air mixtures in a constant volume combustion bomb. The model is based on a one-dimensional three-zone thermodynamic model that calculates the mass transfer and diffusion and the heat transfer between zones. The chemical process involved in the combustion is solved by an in-house chemical kinetics solver with an established reduced hydrogen-oxidation mechanism from literature. The effects of flame stretch and instabilities are simulated using existing experimental data. The calculated laminar burning velocities are compared to existing empirical correlations and experimental data obtained from constant volume combustion bomb tests. The model is able to simulate laminar burning velocities and have the potential to be integrated into IC engine models in the future

Analyzing eventual leader election protocols for dynamic systems by probabilistic model checking

Leader election protocols have been intensively studied in distributed computing, mostly in the static setting. However, it remains a challenge to design and analyze these protocols in the dynamic setting, due to its high uncertainty, where typical properties include the average steps of electing a leader eventually, the scalability etc. In this paper, we propose a novel model-based approach for analyzing leader election protocols of dynamic systems based on probabilistic model checking. In particular, we employ a leading probabilistic model checker, PRISM, to simulate representative protocol executions. We also relax the assumptions of the original model to cover unreliable channels which requires the introduction of probability to our model. The experiments confirm the feasibility of our approach

Structural and biochemical insights into small RNA 3' end trimming by Arabidopsis SDN1.

A family of DEDDh 3'→5' exonucleases known as Small RNA Degrading Nucleases (SDNs) initiates the turnover of ARGONAUTE1 (AGO1)-bound microRNAs in Arabidopsis by trimming their 3' ends. Here, we report the crystal structure of Arabidopsis SDN1 (residues 2-300) in complex with a 9 nucleotide single-stranded RNA substrate, revealing that the DEDDh domain forms rigid interactions with the N-terminal domain and binds 4 nucleotides from the 3' end of the RNA via its catalytic pocket. Structural and biochemical results suggest that the SDN1 C-terminal domain adopts an RNA Recognition Motif (RRM) fold and is critical for substrate binding and enzymatic processivity of SDN1. In addition, SDN1 interacts with the AGO1 PAZ domain in an RNA-independent manner in vitro, enabling it to act on AGO1-bound microRNAs. These extensive structural and biochemical studies may shed light on a common 3' end trimming mechanism for 3'→5' exonucleases in the metabolism of small non-coding RNAs

Onset of cellular instabilities in spherically propagating hydrogen-air premixed laminar flames

Using high-speed Schlieren and Shadow photography, the instabilities of outwardly propagating spherical hydrogen-air flames have been studied in a constant volume combustion bomb. Combustion under different equivalence ratios (0.2 w 1.0), temperatures (298 K w 423 K) and pressures (1.0 bar w 10.0 bar) is visualized. The results show that flames experience both unequal diffusion and/or hydrodynamic instabilities at different stages of propagation. The critical flame radius for such instabilities is measured and correlated to the variations of equivalence ratio, temperature and pressure. Analysis revealed that equivalence ratio affects unequal diffusion instability via varying the Lewis number, Le; increased temperature can delay both types of instabilities in the majority of tests by promoting combustion rate and changing density ratio; pressure variation has minor effect on unequal diffusion instability but is responsible for enhancing hydrodynamic instability, particularly for stoichiometric and near-stoichiometric flames

Locally Spontaneous Dynamic Oxygen Migration on Biphenylene: A DFT Study

The dynamic oxygen migration on the interface of carbon materials, such as graphene and carbon nanotube, has opened up a new avenue to realizing the dynamic covalent materials. However, the understanding of dynamic behaviors of oxygen groups on the non-honeycomb structure, such as the biphenylene sheet, is still limited. Using both density functional theory calculations and ab initio molecular dynamics simulations, we demonstrate that the oxygen groups on the biphenylene, which is an allotrope of graphene and composed of four-, six- and eight-membered rings with unequal C-C bonds, can exhibit locally spontaneous dynamic oxygen migration through the breaking/reforming of the C-O bond. The density of state analyses show that the p-band center of the oxygen atom is closer to the Fermi energy level on biphenylene, compared to that of the oxygen atom adsorbed on graphene. This contrast confirms the locally spontaneous dynamic activity of the oxygen atom on biphenylene. This work provides scientific guidance for the exploration of the locally/globally spontaneous dynamic covalent materials and adds a new member to the 2D dynamic covalent material family.Comment: 13 pages, 4 figure